Search Results (31 total)

Fast proportional rf control is used as the basis for rf field regulation in actual linear accelerator projects like the international linear collider (ILC) and the European x-ray free electron laser (XFEL) based on TESLA technology. Additional control loops improve the field regulation by treating repetitive effects and compensating the beam loading. Nevertheless, the ability for high gain operation of the fast loops is desirable for the strong suppression of nonpredictive and nonrepetitive disturbances. TESLA cavities host nine fundamental modes (FMs) where only one is used for beam acceleration. The unwanted FMs have a significant influence on the proportional rf control loop stability at high gains. Within this paper, the stability of proportional rf control loops taking the FMs and digitalization effects into account will be discussed in detail together with measures enabling a significant increase of the gain values.

We study the phase stability of the Edwards-Anderson spin glass model by analyzing the domain-wall energy. For a bimodal ±J distribution of bonds, a topological analysis of the ground state allows us to separate the system into two regions: the backbone and its environment. We find that the distributions of domain-wall energies are very different in these two regions for the three-dimensional (3D) case. Although the backbone turns out to have a very high phase stability, the combined effect of these excitations and correlations produces the low global stability displayed by the system as a whole. On the other hand, in two dimensions (2D) we find that the surface of the excitations avoids the backbone. Our results confirm that a narrow connection exists between the phase stability of the system and the internal structure of the ground state. In addition, for both 3D and 2D we are able to obtain the fractal dimension of the domain wall by direct means.

We present dc magnetization experiments performed on the doped, polycrystalline system La_0.67−xA_xCa_0.33MnO₃ (A=Ce,Y). The ceramic samples have been obtained by the nitrate decomposition route and structurally characterized by powder x-ray diffraction. Hysteresis loops reveal an exciting and nonrepetitive multistep structure of Barkhausen-like jumps in the low fields region, up to 50 Oe. We discuss our results in terms of a collective response of magnetic moments pinned by frustration due to the competition among different exchange interactions present in the compounds. A simple Ising Hamiltonian with in-plane ferromagnetic interactions and randomly mixed off-plane interactions is introduced to model this hypothesis. Computer simulations based on this model allow us to reproduce most of the features observed in the hysteresis curves, allowing a qualitative understanding of the complex magnetic behavior of this family of manganites.

Without specific counter measures, the LHC type beam in the SPS suffers from longitudinal coupled bunch instabilities. To prevent them, the SPS impedance has been decreased over the last few years and the operation of a high frequency Landau damping system has been established. In the absence of this Landau damping system one may alternatively introduce an rf amplitude modulation to stabilize the beam. We present results obtained by this method in the SPS and considerations for a potential increase of the longitudinal beam stability in the LHC.

The Jahn-Teller effect is invoked to explain the fine structure (isolated zero-phonon lines) observed in both the infrared emission and absorption spectra of substitutional Cr²⁺ impurities in ZnSe and ZnS. The ground ⁵D₂ term of Cr²⁺ is split by crystal field into a ⁵T₂ ground multiplet and an excited ⁵E multiplet. We look at transitions among levels belonging to these two multiplets, which happen to be in the near infrared region. Spin-orbit and spin-spin interactions are taken into account. The Jahn-Teller coupling is introduced as a linear coupling considering both ϵ and τ₂ phonons. The Lanczos-recursion procedure with a proper choice of the initial state is used to calculate the vibronic functions and energies. It is found that ϵ modes only lead to intensities that do not agree well with those of the zero-phonon doublet observed both in emission and absorption in the cases of ZnS and ZnSe, while τ₂ modes give a good explanation of transition energies and transitions strengths in the same cases. A discussion of the relatively high strength of the vibronic coupling for Cr in comparison with other impurities in the same compounds is also included.

Anisotropy is added to the Edwards-Anderson model in such a way that interactions along the x axis are stronger by a factor f with respect to other interactions. Hysteresis cycles for square and cubic ±J Ising spin glasses are obtained by Monte Carlo simulations. Concentration x of ferromagnetic interactions (−J), temperature T, and f are varied to study their effects on the characteristics of the hysteresis loops. Several behaviors are simulated and compared to experimental curves, finding similarities. Important aspects such as virgin curve, remnant magnetization, and coercive field are discussed in detail. It is found that anisotropy tends to stabilize spin-glass phases, leading to a larger remnant magnetization and larger coercive field.

A dynamical Jahn-Teller effect has been proposed to interpret the fine structure of the luminescence spectra of V²⁺ impurities in ZnS and ZnSe. The ground ⁴F term of the impurity is split by the crystal field into three multiplets. The spin-orbit and the spin-spin interactions are taken into account as well as a linear Jahn-Teller coupling with a trigonal phonon mode, both on the ground ⁴T₁ multiplet and on the excited ⁴T₂ multiplet. The Lanczos-recursion procedure with a proper choice of the initial state is followed to calculate the vibronic states. A comparison with experimental energies and intensities indicates that a dynamical Jahn-Teller effect plays an important role to explain the fine structure of the luminescence spectra of ZnSe:V²⁺ and ZnS:V²⁺. In the latter, the temperature effects present in the spectra are also accounted for by the resulting energy-level scheme.

We attempt an explanation of the main features of the high-temperature infrared absorption spectra of Fe²⁺ in III-V compounds (GaAs and GaP). We consider a linear Jahn-Teller interaction with two lattice modes of E symmetry, having energies in the range of acoustical and optical phonons, respectively. The upper vibronic states originating from all electronic states of the free ion must be considered to cope with the many possible transitions that arise at temperatures that populate several low-energy vibronic levels. We use Lanczos-recursion procedures to find energies and wave functions. A comparison with experimental energies and intensities is performed. A discussion comparing present findings with previous results based only on cold lines is also done.

Coupling of acoustical and optical modes is introduced to interpret zero-phonon lines in extended absorption spectra of Fe²⁺ in binary compounds of local symmetry T_d. Both cubic II-VI (CdTe, ZnTe, ZnSe, ZnS) and cubic III-V (GaAs, InP, GaP) compounds are included in analysis and calculations. For the case of ZnS:Fe²⁺, which plays an important role here, interesting experiments are reported. The interpretation of the low-temperature absorption spectra of the seven systems unfolds generalities so all observed lines, as well as the absence of some expected lines, can be identified in the same generic way. In fact, only one parameter is freely varied, which is the coupling constant to one optical mode (additional to the usual acoustical one) which is necessary to explain high-energy lines. The general and consistent explanation of several lines for seven different systems provides a complete picture which allows a deep understanding of vibronic coupling to Fe²⁺ in binary compounds. The values of coupling constants explaining the experiments are tabulated.

Free energy for random energy model is obtained for different values of parameter q defined in nonextensive statistical mechanics. System is found either in paramagnetic or spin-glass phases depending on the value of q.

Three different and independent methods are used to find eigenfunctions and eigenvalues for a linear Jahn-Teller Hamiltonian. They are the following: diagonalization on a Born-Oppenheimer basis developed in the adiabatic limit, diagonalization on a Glauber states basis developed in the strong-coupling limit, and construction of the eigenfunctions by means of the Lanczos method. We explore the space of interactions aiming at the intermediate-coupling limit, finding the regions of best convergence for each method. Comparison among the three methods in terms of their numerical results for energy and expected optical transitions leads to regions of total and partial agreement. Conditions for several zero-phonon lines are discussed. The dominant line is not always the threshold line and it is determined by a nontrivial balance involving all interactions. Conclusions are focused toward finding reliable methods for the different regions of the parameter space. Preparation is done for applications of this approach to explaining optical spectra of magnetic impurities in crystals under different coupling regimes.

The magnetic hysteresis of ±J Ising lattices is analyzed performing a zero-temperature random-walk minimizing energy. A steplike structure presenting a loop divided in four sections is observed. It is shown by Monte Carlo calculations that this structure is rounded off as temperature increases until a thin S shape is obtained, which is in general agreement with experimental results. A simple explanation for this form of hysteresis is given supporting universality and size independence.

±J Ising lattices are exactly solved, finding all ground states. Each original lattice leads to a correlated diluted lattice when all bonds that frustrate in any of the ground states are removed. Correlation appears in the problem when it is found that diluted lattices generated in this way present regions of unfrustrated bonds complemented by regions of frustrated (removed) bonds. The spectral distribution of sizes for unfrustrated regions is not trivial, presenting strong modulations. Here, we characterize such distribution for lattices between 16 and 64 spins, comparing with the monotonic distribution obtained for randomly generated diluted lattices. We deal with results for at least 500 samples in each size. About 2/3 of the samples percolate independently of lattice size; percolation threshold was estimated at 0.41 for these correlated diluted lattices. Most of the samples possess a large unfrustrated region containing about 40% of the bonds, where all spins remain fixed in any of two states with opposite spin orientations. Such a robust way of breaking ergodicity leads to high values in the site order parameters, which implies partial or local spin-glass behavior.

Second-nearest-neighbor interactions are added to the usual nearest-neighbor Ising Hamiltonian for square lattices in different ways. The starting point is a square lattice where half the nearest-neighbor interactions are ferromagnetic and the other half of the bonds are antiferromagnetic. Then, second-nearest-neighbor interactions can also be assigned randomly or in a variety of causal manners determined by the nearest-neighbor interactions. In the present paper we consider three causal and three random ways of assigning second-nearest-neighbor exchange interactions. Several ground-state properties are then calculated for each of these lattices:energy per bond ε_g, site correlation parameter p_g, maximal magnetization μ_g, and fraction of unfrustrated bonds h_g. A set of 500 samples is considered for each size N (number of spins) and array (way of distributing the N spins). The properties of the original lattices with only nearest-neighbor interactions are already known, which allows realizing the effect of the additional interactions. We also include cubic lattices to discuss the distinction between coordination number and dimensionality. Comparison with results for triangular and honeycomb lattices is done at specific points.

The infrared emission spectra of ZnS:V⁺ and ZnSe:V⁺ are reported examining temperature dependence. The unusually rich spectra are theoretically explained by assuming a vibronic coupling due to ε modes. A diagonalization of the Hamiltonian matrix is performed using wave functions constructed in the Born-Oppenheimer limit. The Jahn-Teller energy is the only adjustable parameter. When this parameter is about 80 cm^-1 for ZnS:V⁺ and 65 cm^-1 for ZnSe:V⁺ good agreement between theory and experiment is found and the main features of the spectra for both systems are explained. The frequencies of the coupling modes are taken close to the TA(L) modes. © 1996 The American Physical Society.

Samples of crystalline CdTe doped with two different concentrations of iron were prepared by the vertical high-pressure Bridgman method. Absorption and emission spectra were recorded at liquid-helium temperature in the region of the ⁵T₂(D)? ⁵E(D) infrared transitions of substitutional Fe²⁺(d⁶) ions. Especially in the range between 2200 and 2300 cm^-1, a rich structure is resolved comprising more lines than predicted from plain crystal-field theory. The explanation of all the important lines is found after introducing a vibronic Jahn-Teller term to the Hamiltonian. A linear coupling between the double-degenerate vibrational mode ε (or γ₃) to the electronic orbitals of the atomic multiplet of symmetry ⁵D leads to the diagonalization of the total Hamiltonian in a set of vibronic functions. Just one free parameter is used in the adjustment: the so-called Jahn-Teller energy representing the strength of the coupling. The corresponding value that we report here is 3 cm^-1. The energies thus found are in good agreement with the positions of the observed lines in the spectra. With the final wave functions we can calculate the relative intensities of the most important transitions and approximate theoretical line shape. This is also in good agreement with the experiment. Using these same energies and wave functions a calculation was performed to explain data existing in the literature about far-infrared absorption for the system CdTe:Fe²⁺. Again, good agreement between experiment and theory is found.

Small Ising lattices with both ferromagnetic (F) and antiferromagnetic (AF) exchange interactions (or bonds) and increasing numbers of spins are studied by means of two independent methods: computational solutions to the Hamiltonian problem and topological counting of frustration paths. Equal magnitudes and concentrations are assumed for both types of bonds. Two different geometries are considered: square lattices (SL’s) with coordination number 4 and triangular lattices (TL’s) with coordination number 6. Two-dimensional samples with a total number of spins N between 4 and 64 are considered for SL’s, while N is varied between 4 and 44 for TL’s. They are distributed in two-dimensional arrays where periodic boundary conditions are imposed. After an array is selected, bond distributions (samples) are independently and randomly generated in fixed positions. The physical parameters are then calculated exactly for each sample. The emphasis here is on the ground-state properties and their dependence with size and shape for the two kinds of lattices. All magnitudes correspond to a basic statistics over a large number of samples for each array. The following magnitudes are reported: ground-state energy per bond, frustration segment, abundance of first excited states, remnant entropy, low-temperature specific heat, and site order parameters q, p, and h. Parameters p and h are introduced here, showing advantages over other similar magnitudes. The results are in good correspondence with analytic studies for the thermodynamic limit. This means that the spin site correlation (p) tends to vanish as N grows. However, we have found that the shape dependence modulates the behavior of these systems toward the thermodynamic limit. There is no tendency to vanish for the bond correlation parameter (h). For both kinds of lattices h might be a constant independent of size and shape.

The shapes of the luminescent lines corresponding to transitions in the localized iron impurities of the system InP:Fe²⁺ are studied and adjusted. The starting point of the analysis is the set of wave functions obtained after considering the vibronic coupling. Six different experimental spectra available in the literature are considered. Gaussian shapes are found to explain better transitions to zero-phonon lines, while Lorentzian shapes are better for explaining lines that involve more vibronic mixing. The vibronic coupling by means of a linear Jahn-Teller Hamiltonian fully explains the general structure of the spectra.

A method based on generalized Glauber states is developed to deal with the Jahn-Teller effect on magnetic impurities in II-VI semiconductors, and is successfully applied to ZnTe:Fe²⁺. The method is also applied to ZnS:Fe²⁺ which reaches stability for a lower number of vibrational quanta N, in order to have a basis for comparison with another method. It is found that the present method is more powerful than a previous one based on Born-Oppenheimer functions. The low-energy vibronic functions remain as stable solutions for all values of the coupling parameter. In the case of ZnTe:Fe²⁺, good agreement with both experimental results and previous theoretical calculations is obtained for a Jahn-Teller energy of 250 cm^-1. In the case of ZnS:Fe²⁺ where no manifestation of vibronic coupling has been observed, an upper limit for this energy is found. Possible extensions of this work and their expected difficulties are also discussed.

We investigate the supercooling of a nematic liquid crystal using fluctuating nonlinear hydrodynamic equations. The Martin-Siggia-Rose formalism [Phys. Rev. A 8, 423 (1973)] is used to calculate renormalized transport coefficients to one-loop order. Similar theories for isotropic liquids have shown substantial increases of the viscosities as the liquid is supercooled or compressed due to feedback from the density fluctuations which are freezing. We find similar results here for the longitudinal and various shear viscosities of the nematic phase. However, the two viscosities associated with the nematic-director motion do not grow in any dramatic way; i.e., there is no apparent freezing of the director modes within this hydrodynamic formalism. Instead a glassy state of the nematic phase may arise from a ‘‘random-anisotropy’’ coupling of the director to the frozen density.

The Jahn-Teller coupling between local vibrations of the host III-V semiconductors and the electronic orbitals of Fe²⁺ substitutional impurities are studied from a theoretical point of view. Coupling to both multiplets resulting after crystal-field splitting is considered. Calculations are performed with just one adjustable parameter, namely, the Jahn-Teller energy (E_JT). The compounds GaP:Fe²⁺, GaAs:Fe²⁺, and InP:Fe²⁺ are fully discussed, especially the latter. Other III-V semiconductors such as GaSb:Fe²⁺, InAs:Fe²⁺, and InSb:Fe²⁺ are referred to for particular applications of the model. The results for the lower multiplet show good agreement for the predicted lines with luminescence spectra; the coupling phonon is identified as belonging to the points TA(L) of the Brillouin zone; values for E_JT are about 8 cm^-1 for this coupling. The coupling to the upper multiplet is described in terms of upper limits as the available infrared-absorption spectra do not show evidence for more than one zero-phonon line. A comparison with similar calculations for II-VI compounds with Fe²⁺ as substitutional impurity is also performed.

CuO₂ planes are modeled by means of a square lattice with antiferromagnetic coupling provided by the oxygen between two copper ions and ferromagnetic coupling when the oxygen is absent. The magnetic interaction is described using an Ising Hamiltonian with interacting first-nearest neighbors. The ferromagnetic exchange interaction is supposed to be larger than the antiferromagnetic one. A Monte Carlo routine is then defined in order to minimize the energy and to calculate physical parameters such as correlation to nearest neighbors, time correlation to the same site, and a schematic of a phase diagram.

Theoretical calculations based on a linear Jahn-Teller Hamiltonian are performed for the case of ZnTe:Fe²⁺. The results show a clear and rather strong vibronic coupling, which allows a good interpretation of the available experimental results. A brief study of the line shape calls for experiments with slightly better resolution than presently available from the literature. The reported values for both the Jahn-Teller energy and the frequency of the coupling phonons are in good agreement with those calculated for similar systems. Vibronic functions with vibrational quanta N up to 14 were used in order to study the stability of the solutions. This analysis is also extended to CdTe, ZnS, and ZnSe with iron impurities where results with only N=10 were available.

Antiferromagnetic ground-state energies for infinite Heisenberg linear chains with up to third-nearest-neighbor interactions have been determined by means of numerical extrapolations. On the other hand, new conditions to produce a ferromagnetic ground state have been established for finite chains. These conditions are expressed as lower bounds for the coupling constants. The particular case in which the coupling constant for third-nearest-neighbor interactions is zero is compared in two different ways with previous calculations by other authors, showing good correspondence.